The vascular system is a prime mediator of homeostasis, playing key roles in the maintainence of normal physiologic functioning. For example, the vascular endothelium's barrier function serves to regulate the entry of fluid, electrolytes, and proteins into tissues, blood vessel tone contributes to the regulation of tissue perfusion, and the vascular endothelium's low mitotic index contributes to the regulation of tissue growth. The term “vasculostasis” refers to the maintenance of this homeostatic vascular functioning, and “vasculostatic agents” as agents that seek to address conditions in which vasculostasis is compromised by preventing the loss of or restoring or maintaining vasculostasis.
Compromised vasculostasis has serious pathologic consequences. For example, if vascular permeability increases beyond manageable levels, the resulting edema may negatively impact tissue and organ function and ultimately survival. Examples where excessive vascular permeability leads to particularly deleterious effects include pulmonary edema, cerebral edema, and cardiac edema (Ritchie AC: Boyd's Textbook of Pathology. London Lea and Febiger, 1990). In general, however, edema in any tissue or organ leads to some loss of normal function, and therefore to the risk of morbidity or even mortality. Similarly, excessive endothelial proliferation may damage tissues (such as the retina in proliferative retinopathies) or fuel unwanted tissue growth (such as with tumor growth).
Many pathologic and disease situations are marked by multiple disregulations in vasculostasis. Angiogenesis, for example, encompasses both enhanced vascular proliferation and permeability, as newly-formed blood vessels do not generally exhibit the same level of vascular barrier function as well-established or mature vessels. Examples of such hyperpermeable vasculature can be found in cancers, vasculoproliferative diseases, retinal diseases, and rheumatoid arthritis. The connection between angiogenesis and hyperpermeability may partly result from the dual action of factors such as vascular endothelial growth factor (VEGF), which induces both endothelial proliferation and vascular permeability. This connection may also reflect the immature nature of angiogenic vessels, in which the intracellular and/or extracellular structures or mechanisms that establish normal vascular barrier function have not yet fully formed. It may also be the case that angiogenesis and vascular permeability are linked by a co-dependence on common cellular mechanisms, for example in the case of cellular junction disassembly which would serve to enhance both paracellular permeability and cellular migration (both being components of the angiogenic process). A comprehensive treatment for many diseases, then, might involve vasculostatic agents that act upon one or more components of vasculostasis disregulation (based, for example, upon their level of action along intracellular signaling cascades). One such example would be a single therapeutic agent that impacts both angiogenesis and vascular permeability.
One way of impacting vasculostasis is by influencing endothelial cell responses to environmental signals (such as hypoxia) or vasoactive agents. For example, the vascular endothelium regulates fluid balance by adjusting both transcellular permeability (movement of fluid and proteins across endothelial cells via a network of vesicles) and paracellular permeability (movement of fluid and proteins between inter-endothelial cell junctions). Edema is most commonly thought to result from a breakdown in the inter-endothelial cell barrier, leading to increased paracellular permeability at the capillary and postcapillary venule level. Mechanistically, paracellular vascular leakage results from a breakdown in inter-cellular junctional integrity, via the dissolution of tight junctions and coupled to changes in cytoskeletal support elements that maintain normal cell-to-cell apposition. Several vasoactive mediators can trigger dissolution of these cellular elements, including histamine, bradykinin, thrombin, nitric oxide, eicosanoids (e.g., thromboxanes and leukotrienes), platelet activating factor (PAF), tumor necrosis factor (TNF), interleukins (e.g., IL-1 and IL-6), hepatocyte growth factor (HGF), and vascular endothelial growth factor (VEGF). Using VEGF as an example, the sequence of events that lead to vascular leakage is generally believed to be as follows: reduced blood flow (e.g., as a result of thrombus formation) leads to tissue hypoxia, which leads to the upregulation of VEGF production, which leads to induction of vascular leakage. This VEGF effect is at the level of the endothelial cell, in other words VEGF binding to specific VEGF receptors expressed on endothelial cells leads to a cascade of intracellular events culminating in the loss of normal intercellular barrier function. Therefore, by affecting these intracellular events, vasulostatic agents could counter the negative effects of environmental signals such as hypoxia or vasoactive mediators such as VEGF, and thereby work to restore vasculostasis.
The cascade of events that leads to the loss of endothelial barrier function is complex and incompletely understood. Data support a role for kinases as at least one aspect of this process. For example, VEGF-mediated edema has been shown to involve intracellular signaling by Src family kinases, protein kinase C, and Akt kinase. Kinases are believed to mediate the phosphorylation of junctional proteins such as beta-catenin and vascular endothelial (VE)-cadherin, leading to the dissolution of adherens junctions and the dissociation of cadherin-catenin complexes from their cytoskeletal anchors. In addition, proteins which regulate the intercellular contractile machinery such as myosin light chain kinase (MLCK) and myosin light chain (MLC) are also activated, resulting in cellular contraction, and therefore an opening of intercellular junctions.
Maintaining or restoring vasculostasis should be beneficial to overall patient outcome in situations such as inflammation, allergic diseases, cancer, cerebral stroke, myocardial infarction, pulmonary and cardiac insufficiency, renal failure, and retinopathies, to name a few. In addition, edema formation is a recognized but unwanted consequence of many therapeutic interventions, such as immunotherapy, cancer chemotherapy and radiation therapy, therefore vasculostatic agents that inhibit vascular permeability could be used in a co-therapy approach to reduce the deleterious side-effects of such therapies. Furthermore, in many cases edema formation causes uneven delivery of therapeutic agents to diseased tissues, therefore vasculostatic agents that inhibit vascular permeability could be used in a co-therapy approach to enhance delivery and efficacy of such therapies. Finally, as edema is a general consequence of tissue hypoxia, it can also be concluded that inhibition of vascular leakage represents a potential approach to the treatment of tissue hypoxia. For example, interruption of blood flow by pathologic conditions (such as thrombus formation) or medical intervention (such as cardioplegia, organ transplantation, and angioplasty) or physical trauma, could be treated both acutely and prophylactically using vasculostatic agents that reduce vascular permeability.